Fresnel solar collector arrangement

ABSTRACT

The invention relates to a Fresnel solar collector arrangement consisting essentially of a receiver ( 1 ) and a mirror arrangement associated with the receiver ( 1 ). The arrangement is temperature-compensated by the use of materials with the same temperature expansion coefficient for the receiver mast ( 2 ) and the mirror supporting framework ( 4 ), and the adjustment of the primary mirror ( 6, 6 ′) in relation to the sun by a mechanical coupling of the mirror is simplified by means of an electromotive connecting rod.

The invention relates to a Fresnel solar collector arrangement.

This is understood to mean a line-focusing system in which multiple mirror strips disposed parallel to a receiver are made to track the position of the sun, and the solar radiation is guided onto a fixed absorber tube in which a heat storage medium flows. In addition, a secondary reflector assigned to the absorber tube guides the radiation onto the focal line essentially formed by the absorber tube. The absorber tube and the secondary reflector form the receiver disposed in elevated manner above the mirror strips. Such a Fresnel solar collector is currently in operation in Australia, for example, in a field trial. The heat that is produced can be utilized as process heat, or it can be converted into an electric current, for example by means of a Stirling motor.

The advantage of Fresnel solar collectors as compared with conventional parabolic trough collectors lies in their significantly simplified structure. Parabolic trough collectors consist of a reflector that has the shape of a parabolic cylinder. Here also, the light is focused onto a line, the focal line. The absorber tube of the parabolic trough collector, which absorbs the concentrated radiation and passes it on to the medium flowing through, is situated in this line. In this connection, the medium is typically heated to values of approximately 400° C. In order to improve the efficiency, the absorber can be surrounded by a glass tube. A vacuum prevails in the interstice between absorber tube and glass tube, for insulation. The “solar steam” produced in this way can also be utilized directly for process heat applications, or for conventional steam power plants and cogeneration power plants.

Alternatively, flat collectors and CPC collectors are known as further types of collectors.

In this connection, the efficiency of the Fresnel solar collector essentially depends on how well the reflected solar radiation is focused onto the absorber tube. For this purpose, it is practical to make the primary mirrors assigned to the absorber tube track the sun. Only in this way can acceptable efficiencies be achieved for the system. This usually takes place by means of an electric motor assigned to each primary mirror. In turn, the electric motors are usually provided with a timing device, so that tracking is more a question of controlling than of regulating.

A significant problem with Fresnel solar collectors is that such arrangements achieve their best efficiencies in regions with the highest incoming solar radiation, for example in desert regions, where extreme temperature variations from degrees below zero to degrees above zero of far in excess of 40° C. are at least not unusual. The materials and supporting structures used are exposed to considerable stresses, in this connection, whereby thermal deformations of the material are practically unavoidable and therefore can lead to angular deviations within the entire system, which can be manifested in the double-digit percentage range in the efficiency of the entire system. Even a small angle deviation in the supporting structure of the mirror arrangement can lead to having a large part of the radiation reflected by the primary mirrors not focused onto the absorber tube but rather reflected past the absorber tube. Furthermore, individual control, i.e. individual regulation and coordination of the various electric motors for tracking, i.e. controlling the panning movement of the mirrors is accompanied by considerable regulation and control effort, making the system somewhat susceptible to malfunctioning.

Proceeding from this prior art, the invention is based on the task of configuring the system more robustly, overall, and of improving its efficiency as much as possible.

This task is accomplished by means of a Fresnel solar collector arrangement according to the main claim. Advantageous embodiments can be seen in dependent claims 2 through 11.

Because, according to the main claim, the mirror supporting framework is mounted in a fixed position in the region of the receiver supporting framework and/or in connection with the receiver supporting framework, and furthermore is mounted in sliding manner, i.e. free from constraint, it is assured, in the case of unavoidable thermal expansion of the supporting framework as a result of the effects of heat, that the mirror supporting framework balances out these corresponding changes.

This succeeds even better if the receiver supporting framework and the mirror supporting framework are essentially made from the same material and are essentially mounted in fixed manner at the same location. If thermal expansions or contractions of the material occur, one can at least approximately assume that the alternating expansions of the supporting frameworks take place to the same extent. For example, the receiver mast is then expanded, as the result of the effect of heat, in approximately the same way as the mounting rails of the primary mirrors disposed as the mirror framework. Because the receiver framework and the mirror supporting framework are at least essentially disposed orthogonal relative to one another and are made from the same material, and therefore have the same expansion coefficient, it is assured that the angle relationships do not change relative to one another or, at most, change only slightly. However, this is possible only if both the receiver framework and the mirror supporting framework are mounted in constraint-free manner, i.e. only one of at least two required supports is fixed in place. This surprisingly simple solution eliminates complicated re-adjustments for material expansions or contractions, or making a largely hopeless attempt to use materials that are more or less temperature-independent. The use of such materials is usually ruled out for cost reasons alone.

In a concrete embodiment, the receiver of the Fresnel solar collector arrangement can be mounted as an absorber tube on a row of receiver masts, whereby the mirror supporting framework can also be mounted in a fixed location at the same point, if necessary using the same concrete pedestal. In this connection, receiver mast and mirror supporting framework are advantageously made from steel 37, in each instance, and therefore exhibit largely the same expansion coefficient.

In an advantageous further development, some of the primary mirrors mounted on the mirror supporting framework are combined to form a primary mirror group, which in turn are mechanically coupled by means of a common mechanical setting element, for tracking purposes, and thus are made to track the sun. Because of the use of a common setting element, complicated coordination, complicated control and regulation of the electric motors used is eliminated, at least within the primary mirror group in question. Instead, the entire primary mirror group can be adjusted by means of a common setting element, whereby the relative angle relationship between the primary mirrors is maintained at all times. Again, this is based on the actually trivial recognition of the law governing radiation, that the relative angle adjustments of the primary mirrors required in the course of tracking the sun, where these mirrors are disposed one behind the other, in an imaginary orthogonal line relative to the absorber tube, which is disposed at a distance and elevated, are the same relative to one another. Incidentally, this would also apply to the primary mirrors disposed on an imaginary line parallel to the absorber tube.

In this connection, the embodiment of the collector arrangement explained above, with mechanical coupling for the common panning movement of the primary mirrors by means of a common setting element, is also advantageous independent of the constraint-free mounting of the mirror supporting framework.

This common panning movement is achieved as a result of connecting the primary mirrors of a primary mirror group by means of a tracking shaft. Due to the movement of the connecting rod when aligning the primary mirrors, a rotation of the tracking shaft is brought about, which is uniformly transferred to the entire primary mirror group by means of the connection.

It is advantageous if the tracking shaft is mounted, at regular intervals, in roller bearing blocks that surround the shaft but that only support it using roller elements. These roller elements permit axial rotation of the tracking shaft, and are formed in barrel-like shape, in other words are essentially cylindrical, whereby their mantle surfaces bulge out. This shape makes it possible to dispose the tracking shaft not only along planar surfaces but also, if required, to guide it along its path over different heights. The shaft can be positioned at a slant on the roller elements, so that simultaneous slanted positioning of the roller bearing block can be eliminated.

In a concrete embodiment, mechanical coupling of the primary mirrors combined to form a group can be implemented by means of a common connecting rod, by way of which the primary mirrors mounted on the mirror supporting framework so as to pivot are pivoted relative to the absorber tube, as a function of the position of the sun, i.e. the time of day or, to say it better, they are made to track the sun.

In an advantageous embodiment, the connecting rod is driven by an electric motor, using a linear motor, whereby the connecting rod, which is disposed orthogonally relative to the longitudinal expanse of the absorber tube, is moved inward or outward, as a function of the sun's position, by means of the linear motor.

In an advantageous embodiment, water vapor or thermal oil flows inside the absorber tube and is heated to a temperature of up to approximately 400° C. by the reflected radiation. The thermal medium heated in this way can then be passed to further use, in known manner, or can be used to produce electricity.

In order to further improve the efficiency of the arrangement despite the improved angular accuracy of the arrangement, a secondary reflector is additionally assigned to the absorber tube, which reflector surrounds the absorber tube essentially like a shield, and thus captures and deflects possible scattered radiation from the primary mirrors, in such a way that this scattered radiation is also focused onto the absorber tube.

Thus the secondary reflector is also disposed so that the absorber tube lies essentially in the focal line of the secondary reflector.

In a further embodiment of the electric motor drive, the linear motor is also disposed essentially centrally, i.e. approximately in the region of the imaginary line formed by the receiver masts disposed in a row. In this connection, when one and the same linear motor is used for turning, one or more primary mirror groups driven by one or more connecting rods, on the left of the absorber tube, and one or more primary mirror groups driven by one or more connecting rods, on the right of the absorber tube, can be driven in such a way that a time-controlled panning movement of the primary mirrors, i.e. a panning movement that tracks the sun, takes place relative to the absorber tube.

The necessarily opposite movement of the primary mirrors on the right of the absorber tube in comparison to the primary mirrors on the left of the absorber tube is implemented by means of a deflection mechanism for the linear movement of the connecting rod, assigned to only one of the two sides.

In an advantageous embodiment, the linear motors can be connected to a common control and/or regulation unit, since the relative movements to be carried out by the connecting rods are exactly identical over the entire length of the absorber tube, and thus common regulation is possible for the entire system.

The invention will be explained in greater detail below, using an exemplary embodiment only shown schematically in the drawings.

The drawing shows:

FIG. 1: a Fresnel solar collector arrangement in cross-section,

FIG. 2: a detail of the Fresnel solar collector arrangement in a schematic diagram, and

FIG. 3: a control diagram for the Fresnel solar collector arrangements shown in FIGS. 1 and 2.

According to the illustration in FIG. 1, the Fresnel solar collector arrangement consists of a receiver 1 mounted on a receiver mast 2. For this purpose, the receiver mast 2 is mounted in a fixed bearing 3 that simultaneously represents the center axis of a mirror supporting framework 4 disposed with angle symmetry. In this connection, the mirror supporting framework 4 essentially consists of supporting rails 5 made from the same material as the receiver mast 2, namely steel 37 in the case of the present exemplary embodiment, and extend orthogonally outward, in each instance, from the longitudinal axis of the receiver 1. In this connection, the receiver 1 essentially consists of an absorber tube in which a thermal medium that acts as a heat storing material flows. This can be simple steam or a thermal oil. The absorber tube is generally surrounded by a secondary reflector that captures any stray radiation of the mirror arrangement and deflects it onto the absorber tube. The primary mirrors 6, 6′ are mounted to pivot on both sides of the supporting structure, i.e. essentially with mirror symmetry, on mirror paths relative to the receiver 1 set up in elevated manner. In this connection, the mirror paths are mounted on the mirror supporting framework 4 essentially in such a way that the solar radiation acting on the Fresnel solar collector arrangement is reflected and deflected in such a way that it is focused onto the absorber tube in the region of the receiver 1. Ideally, the absorber tube forms the focal line of the primary mirrors 6, 6′ mounted on the mirror supporting framework 4. In this connection, several primary mirrors 6, 6′ are to each receiver 1 at a different distance, i.e. at an increasing orthogonal distance from the central axis of the mirror supporting framework 4 defined by the absorber tube.

Relative to the setup base, the mirror supporting framework 4 itself is, in turn, mounted with foot elements 7 connected only by means of slide bearings to the supporting rails 5, which extend in fixed manner, orthogonal relative to the longitudinal expanse of the receiver 1. Thus, in concrete terms, the receiver mast 2 and the supporting rails 5, which are disposed one behind the other in the longitudinal expanse of the receiver 1, are fixed in place only in the fixed bearing 3, and otherwise are mounted in constraint-free manner, so as to slide. Since both the receiver mast 2 and the supporting rails 5 are made from steel 37 and therefore possess essentially identical expansion coefficients, any thermal expansion of the two supporting frameworks is also essentially the same. The longitudinal expansion of the receiver mast 2 is thus essentially compensated in that any angle error in the arrangement, with the possible consequence that the absorber tube moves out of the focal line of the mirror arrangement, is compensated by a similar expansion of the supporting rail 5.

The Fresnel solar collector arrangement according to FIG. 1 is thus essentially temperature-compensated in self-regulating manner, in that any material expansions and contractions resulting from the absolutely normal extreme temperature variations in the regions of use of Fresnel solar collector arrangements are reciprocally balanced out. As a result, the losses due to scattering of the reflected radiation, which have a very negative effect on the yield factor of the system, are avoided to a great extent. Complicated techniques for compensating the changes in length of the materials used, due to temperature, can therefore be eliminated, to a great extent.

According to FIG. 2, the arrangement is advantageously supplemented in that the primary mirrors 6, 6′ assigned to the individual supporting rails 5, are, in each instance, connected with the supporting rail 5, in each instance, by means of a mirror support 8, 8′, so as to pivot. In this connection, it is known from the prior art to assign a separate electric motor to each primary mirror 6, 6′ and to achieve tracking of the primary mirrors 6, 6′ according to the position of the sun relative to the receiver 1, using this electric motor drive. According to FIG. 2, several primary mirrors 6, 6′ are combined to form a primary mirror group that is characterized by being mechanically coupled with one another by means of a common setting element, namely a connecting rod 10, 10′. The connecting rod 10, 10′ is driven in linearly displaceable manner, by an electric motor, using a linear drive 11, whereby the movement of the connecting rods 10, 10′ on the left and right of the receiver mast 2, and thus the movement of the receiver 1, go in opposite directions by means of a deflection mechanism not illustrated further here. The connecting rods 10, 10′ on the left and right of the receiver 1 are thus either both moved inward or both moved outward. This is understood to mean that one of the two connecting rods 10 or 10′ acts only indirectly on the primary mirrors 6, 6′, namely by way of a deflection mechanism that leads to the aforementioned opposite movement. This, in turn, brings about the result that the mirrors disposed on the right and left are turned toward or away from the centrally disposed reflector or absorber tube at precisely the same angle relationship. The solution shown according to FIG. 2 therefore makes it possible to create mechanical coupling by way of a simple connecting rod 10, 10′, using a single electric motor, and thus to eliminate complicated coordination of several individual electric motors, at least along one supporting rail 5, i.e. within a primary mirror group, and, instead, to allow precise tracking following the position of the sun, using a single common linear drive, because of the angle accuracy of the arrangement.

In this connection, according to the schematic diagram in FIG. 3, this can be a control unit and/or a regulation unit. According to the illustration in FIG. 3, a common regulator 12 is assigned to the linear motors 11, 11′, 11″, to which one or more connecting rods 10, 10′ or supporting rails 5 are assigned, in each instance. In the simplest case, this regulator 12 can be controlled in time-controlled manner, in the sense of a control, according to a predefined program that associates every time of day with a certain position of the sun and thus with an angle position of the primary mirrors 6. For this purpose, the regulator 12 is data-connected to a time detection device 14. Alternatively, however, the regulator 12 can also be connected to a true actual value/reference value comparator 13, whereby the actual value and target value either compare the real position of the sun to the target value default or, on the other hand, the efficiency of the system is directly fed back to the regulating variable, for example by evaluating the radiation intensity achieved or the current yield of electricity as the actual value, in order to determine any regulatory deviation. The angle positioning of the primary mirrors 6, 6′ can then be re-adjusted using the setting element. Thus, understood correctly, the connecting rod 10, 10′ more or less represents the setting element for the regulation or tracking of the primary mirror arrangement, whereby the electric motor 11, 11′, 11″ is also part of this setting element. The triggering or regulation of the linear motors 11, 11′, 11″ is implemented by means of a common regulator 12.

FIGS. 4 and 5 show a roller bearing block 15 in which a tracking shaft 17 is guided. The tracking shaft 17 connects the primary mirrors 6, 6′ of a primary mirror group and ensures parallel rotation of all the mirrors of this group as a result of tracking initiated by the movement of a connecting rod 10, 10′. The roller bearing block 15 surrounds the tracking shaft 17, whereby the shaft is mounted on roller elements 16, 16′, 16″ in the roller bearing block 15. These roller elements 16, 16′, 16″ are essentially cylindrical, but have concave mantle surfaces on which the tracking shaft 17 is supported. As a result of this barrel-like shaping, it is possible to position the tracking shaft 17 at a slant as shown in FIG. 5, whereby the roller bearing block 15 remains in its perpendicular position. This allows laying the tracking shaft 17 along slanted surfaces, for example on hills or on uneven terrain. In this connection, one must, of course, ensure that the receiver 1 is not covered up relative to the primary mirrors 6, 6′ in question.

Thus a Fresnel solar collector arrangement is described above, which is temperature-compensated, to a great extent, in that materials having the same thermal expansion coefficients are used for the supporting rails 5 of the mirror supporting framework 4 and the receiver masts 2 and that furthermore, the receiver mast and the mirror supporting framework 4 are mounted in constraint-free manner. Beyond that, tracking of the primary mirrors 6, 6′ to follow the position of the sun is significantly simplified by means of mechanical coupling of the primary mirrors 6, 6′.

LIST OF REFERENCE SYMBOLS

-   1 Receiver -   2 Receiver mast -   3 Fixed bearing -   4 Mirror supporting framework -   5 Supporting rail -   6, 6′ Primary mirror -   7 Foot elements -   10, 10′ Connecting rod -   11 Linear motor -   12 Regulator -   13 Actual value/target value comparator -   14 Time detection device -   15 Roller bearing block -   16, 16′, 16″ Roller elements -   17 Tracking shaft 

1. Fresnel solar collector arrangement having at least one receiver (1) mounted on a receiver supporting framework, elevated relative to several primary mirrors (6, 6′) that are disposed on a mirror supporting framework (4) on both sides of the receiver (1) so as to pivot, in such a manner that the solar radiation reflected by the primary mirrors (6) is focused at least essentially onto the receiver (1), and the primary mirrors (6) are, in each instance, made to track the sun, wherein the mirror supporting framework (4) that extends bilaterally outward from the receiver supporting framework is mounted in a fixed location in the region of the receiver supporting framework and/or in connection with the receiver supporting framework and, for the remainder is mounted to slide, i.e. in constraint-free manner, at least to a great extent.
 2. Fresnel solar collector arrangement according to claim 1, wherein the receiver supporting framework and the mirror supporting framework (4) are made from a material having at least largely identical material expansion coefficients, preferably from steel 37, in each instance.
 3. Fresnel solar collector arrangement according to claim 1, wherein the receiver (1) comprises an absorber tube that is mounted in an elevated manner by means of a row of receiver masts (2), preferably disposed in an imaginary straight line, which essentially form the receiver supporting framework, and that the mirror supporting framework (4) comprises supporting rails (5) that are spaced apart from one another and connected in a framework-like manner by means of appropriate struts, if necessary, whereby these supporting rails (5) extend outward at least essentially orthogonally from an imaginary straight line of the receiver masts (2) disposed in a row, and that these supporting rails (5) are fixed in place in connection with the receiver masts (2), or in the foundation region of the receiver masts (2), and otherwise are mounted to slide.
 4. Fresnel solar collector arrangement according to claim 1, wherein the primary mirrors (6, 6′) disposed on a common supporting rail (5) along an imaginary line parallel or perpendicular to the longitudinal expanse of the absorber tube of the receiver (1), are combined to form a primary mirror group, whereby the primary mirrors (6, 6′) of the primary mirror group are mechanically coupled by means of a common setting element and, as a result, are made to track the sun together.
 5. Fresnel solar collector arrangement according to claim 4, wherein the primary mirrors (6) assigned to one or more supporting rails (5) disposed in an imaginary extension relative to one another are combined to form a primary mirror group.
 6. Fresnel solar collector arrangement according to claim 5, wherein the primary mirrors (6, 6′) of the primary mirror group are, in each instance, mounted to pivot on the supporting rail(s) (5), whereby these primary mirrors (6) are rigidly connected with one another by means of a connecting rod (10, 10′), preferably driven by an electric motor, and can be pivoted together, relative to the absorber tube of the receiver (1), by means of linear movement of the connecting rod (10, 10′) in the direction of the longitudinal expanse of the connecting rod (10, 10′), in terms of their respective angle of incidence.
 7. Fresnel solar collector arrangement according to claim 6, wherein the primary mirrors (6, 6′) of a primary mirror group are firmly connected with one another by means of a tracking shaft (17), for the purpose of common pivoting, which shaft is essentially disposed parallel to the longitudinal expanse of the absorber tube, to rotate relative to the supporting rails (5).
 8. Fresnel solar collector arrangement according to claim 7, wherein the tracking shaft (17) is mounted in a roller bearing block (15) that surrounds the shaft, whereby the shaft is held in the roller bearing block (15) by means of a multiplicity, preferably three, of barrel-shaped roller elements (16, 16′, 16″), in such a manner that axial rotation of the tracking shaft (17) is made possible.
 9. Fresnel solar collector arrangement according to claim 5, wherein the connecting rod (10, 10′) can be displaced by a motor in the direction of its longitudinal expanse, by means of a linear motor (11).
 10. Fresnel solar collector arrangement according to claim 6, wherein the linear motor (11) is preferably disposed in the region of the receiver mast (2), and one or more connecting rods (10, 10′) are driven by the linear motor (11), whereby the primary mirrors (6) disposed on the right of the receiver (1) are moved opposite the primary mirrors (6′) on the left of the receiver (1), for the purpose of tracking, whereby this opposite movement is implemented by means of a corresponding deflection mechanism provided on only one side of the receiver (1).
 11. Fresnel solar collector arrangement according to claim 1, wherein a heat storage medium, preferably steam or thermal oil, flows inside the absorber tube.
 12. Fresnel solar collector arrangement according to claim 10, wherein the receiver (1) additionally comprises a secondary reflector assigned to the absorber tube, which essentially absorbs the scattered radiation reflected by the primary mirrors (6, 6′), and deflects it onto the absorber tube.
 13. Fresnel solar collector arrangement according to claim 1, wherein the dimensions and spacings of the primary mirrors (6, 6′) are dimensioned in such a way that mutual shadowing of the primary mirrors (6, 6′) is precluded, at least to a great extent. 